1. Field of the Invention
The present invention relates to a threshold value control system in a receiver circuit in an ISDN (Integrated Systems Digital Network) terminal or in a network termination in an ISDN network.
2. Description of the Related Art
As a cross reference related to the present invention, Japanese Patent Publication No. 61-227440 is known to the public.
In the receiver circuit in the ISDN terminal or in the network termination, a comparator is conventionally used for comparing an input signal and a threshold voltage so as to discriminate the digital signal level of the input signal. As the methods for determining the threshold voltage, conventionally, there are a fixed method in which the threshold voltage is constant and an ATC (Automatic Threshold Control) method in which the threshold voltage is approximately linearly changed dependent on the voltage of the input signal.
In the prior art, there are problems in that, according to the fixed method, the regulation of a total phase deviation cannot be satisfied and, according to the ATC method, the threshold voltage fluctuates due to noises and so forth when using a point-to-point connection and the threshold voltage fluctuates due to the superimposition of the frame synchronization signals under the multipoint connection, as later described in more detail with reference to the drawings.